The Sensata Dominican REP (or Mexico depending on where made) RP5A is a thermal protector* for lighting fixtures. It has a heater voltage of 120Vac and has low sensitivity. It has four leads, black, white, blue and red. When the temperature of the fixture is exceeded it will cause the light to blink. It is auto resetting for when the fault is corrected. The Sensata RP5A is designed to fit into a 1/2" KO.
*A thermal protector is a safety device designed to prevent lighting fixtures from overheating, which can lead to damage, fire hazards, and reduced lifespan of the fixture.